Metal oxide varistors such as zinc oxide varistors have classically been prepared by conventional ceramic techniques of mixing the metal oxide powders, typically by ball-milling, pressing the mixed powder into the desired shape and sintering to form varistors. It has been recognized that it is desirable to have a homogeneous mixture of the zinc oxide and the additive oxides. Various processes have been used to achieve varying degrees of homogeneity in various metal oxide varistor powders.
In U.S. Pat. No. 4,094,061 to Gupta et al. zinc oxide powder is mechanically mixed with various additive metal oxide powders which is then added to an aqueous binder solution and then freeze dried or spray dried to form a metal oxide varistor powder.
In U.S. Pat. No. 4,142,996 to Wong et al. the metal oxide precursors, such as salts, are precipitated from a solution to form a dehydrated melt, then calcined at temperatures up to 800.degree. C. to form the oxides. The powder is then formed by spray drying.
U.S. Pat. Nos. 4,023,961 to Douglas et al. and 4,416,963 to Takimoto et al. illustrate various processes of spraying drying to obtain powder, including spraying in a burning furnace at 900.degree. C.
In U.K. Pat. No. 1,518,184 to Blake the metal oxides are mixed with a carbohydrate material which is then burned off to leave agglomerates of metal oxide particles.
In Dosch et al. "Chemical Preparation of High Field Zinc Oxide Varistors", Sandia National Laboratories Report SAND85-0195, Technical Information Service, U.S. Dept. of Commerce (Sept. 1985) the metal oxide powders are prepared by coprecipitation as hydrous oxides and converted to oxalates by the addition of oxalic acid. After washing and drying the coprecipitate is calcined at 600.degree. C. to produce the oxide mixture. Bismuth was then added as bismuth metal dissolved in nitric acid and the powders again calcined at 400.degree. C. to produce the final powder which is then pressed into the desired shape and sintered at a temperature in the range of 675.degree.-740.degree. C. to form varistors.
In U.S. Pat. No. 4,575,440 to Palilla a solution is formed from soluble salts of precursors of the additive metal oxides. Zinc oxide powder is added to the solution to form a suspension. This suspension is dried to form a cake or a powder, which is then crushed, sieved and calcined at temperatures up to 800.degree. C. to convert the additive metal oxide precursors and salts to the oxides. The calcined powder is ground, sieved and formed into a slurry which is then spray dried to form the final powder.
In U.S. Pat. No. 4,510,112 to Lauf and U.S. Pat. No. 4,540,971 to Kanai et al. The primary and additive metal oxides are either precipitated from solution separately and then mixed or the primary and additive metal oxides are simultaneously coprecipitated to form the mixture of metal oxide powders. See also Lauf et al. "Fabrication of High-Field Zinc Oxide Varistors by Sol-Gel Processing," Am. Ceram. Soc. Bull. 63(2), 270-81 (1984).
The disclosures of the above references are incorporated herein by reference.
We have not only observed that it is desirable to produce a metal oxide varistor precursor powder in which the average particle size of the primary metal oxide, such as zinc oxide, is relatively small, for example less than about 5 microns, we have determined that the average particle size of the zinc oxide should be significantly larger than the average particle sizes of the additive metal oxide components, for example the ratio of the radius, r, of the average particle sizes of the additive metal oxide components to the radius, R, of the average particle size of the zinc oxide should be less than 0.9.
We have determined that the dispersion or distribution of the particles of smaller additive metal oxides should be uniform among the larger particles of zinc oxide grains, for example there should be at least one particle of an additive metal oxide within a distance of 5R from each zinc oxide particle, where R is the average radius of the zinc oxide particle.
Therefore, it is an object of this invention to provide a metal oxide varistor precursor powder wherein the particles of additive metal oxides are smaller than the particles of zinc oxide and are uniformly distributed among the zinc oxide particles and to provide a process to produce such a metal oxide varistor precursor powder.
We have determined that in a desired range of zinc oxide particle sizes, e.g. between about 0.1 and about 5 microns, the various conventional mechanical and chemical processes and equipment for mixing and dispersing the smaller particles of additive metal oxides throughout the larger particles of zinc oxide are not sufficiently controllable and predictable in normal use to produce the desired uniformity of distribution and dispersion of the particles of additive metal oxides among the zinc oxide particles to produce the desired metal oxide varistor precursor powder. Therefore, it is an object of this invention to provide a controllable alternative to conventional mechanical or chemical means for mixing the additive metal oxides and the zinc oxide.
We have determined that in the varistor structure resulting from sintering the pressed metal oxide varistor precursor powder, it is desirable to have a uniform zinc oxide grain size and a uniform distribution of additive metal oxides among the zinc oxide grains. Therefore, it is an object of this invention to provide such a varistor and means for producing a varistor having the desired uniform grain size properties and distribution of additives after sintering.
We have determined that it is desirable to have in the varistor product, produced by pressing and sintering the metal oxide varistor precursor powder, a uniform grain boundary between zinc oxide grains, which provides a higher volts per grain boundary, and a lower dielectric constant compared to varistors having non-uniform grain boundaries. Therefore, it is an object of this invention to provide a varistor product having uniform grain boundaries between zinc oxide grains.